One common method of locking compressor or turbine blades against axial movement, which is in the direction substantially parallel to the rotary axis of the disk, is accomplished by use of external hooks on the blades and a circular snap ring. The conventional hook and snap ring configuration makes use of a slotted extension, or hook, on one axial edge of the blade to provide a seat for the snap ring. The snap ring is provided with a relaxed diameter that forces it to seat tightly in the slot provided by the hook.
While performing its retention function effectively, the hook and snap ring configuration suffers from a number of disadvantages which have become more prominent in the continuing effort to lower aircraft engine weight and improve aerodynamic efficiency. Firstly, the external hook adds dead weight without helping to maintain aerodynamic efficiency. Such hooks also produce windage during rotation of the blade and disk assembly, which reduces the overall aerodynamic efficiency of the combination.
The external hook and snap ring scheme also poses a substantial risk of blade loss due to hook or snap ring failure. This external hook arrangement is particularly susceptible to foreign object damage (FOD) resulting in blade release. Foreign objects inadvertently passing through the flow stream of a gas turbine or other type of rotary engine may impact this exposed hook and snap ring configuration and result in a catastrophic release of blades. Other common methods of externally locking blades into position upon a rotary disk suffer from similar disadvantages.
A need therefore exists for a blade retention system that reduces dead weight and windage losses associated with external locking arrangements.
A further need exists for a blade retention system that is more resistant to foreign object damage (FOD).